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Net movement of electric charge through a
medium– current
◦ Actually measures rate of charges passing through
a cross-sectional area
◦ I = ΔQ/Δt
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SI Unit: Ampere (A)
◦ 1 A = 1 C/s
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Conventional current defined in terms of
positive charge movement
◦ Actual motion of charges can be positive, negative
or mixture of both
◦ Positive and negative charges in motion: charge
carriers
◦ Conventional current defined as current consisting
of positive charges that would have the same effect
as the actual motion of the charge carriers,
regardless of the actual charge of the charge
carriers
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Charges set in motion by electric fields
◦ Good conductors permit easy motion of charge
carriers
 Metals
 Electrolyte solutions

Charge carriers move fairly slowly through
media
◦ When switch is closed, electric field established
through circuit
◦ Field sets charges in motion throughout wires
◦ Field moves at speed of light, charges move more
slowly

Drift velocity is net velocity of charge carriers
◦ In electrostatic equilibrium, charges move randomly
◦ A potential difference applied through wire
generates electric field
◦ Forces sets charges in motion opposite electric field
and establishes current

Charges do not move in straight lines
◦ Matter of conductor blocks straight path
◦ Charges and vibrating particles create zigzag
pattern in conductor
◦ Energy from charges transferred to conductor
through collisions and increases their kinetic energy
 Conductor’s temperature rises

Energy gained by charges as accelerated
through electric field greater than loss due to
collisions
◦ See net motion in direction opposite the electric
field
◦ Velocity: drift velocity (vdrift)

A potential difference applied to a conductor
sets charges in motion from higher electric
potential to lower electric potential
◦ Potential difference maintains current

Batteries/generators maintain potential
difference across terminals through
conversions of energy
◦ Batteries: chemical to electrical
◦ Generators: mechanical to electrical

Current may be direct or alternating
◦ Direct: charges move in only one direction
◦ Alternating: movement of charges continuously
changes direction

Batteries: each terminal has fixed sign so
charge flows only one direction
◦ Direct current

AC sources: terminals change sign constantly,
so net flow of charge is 0
◦ Slow motions can be seen with flickering of lights
◦ In US, AC operates at frequency of 60 Hz
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Opposition to the motion of a charge through
a conductor
Ratio of potential difference to current
◦ R = ΔV/I
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SI unit: ohm (Ω)
For many materials, resistance is constant
over wide range of potential differences
◦
◦
◦
◦
Ohmic substances
Ohm’s Law = ΔV/I = constant
Δ = IR
Graph of I vs V would be linear

Non-ohmic substances: resistance not
constant over voltage range
◦ Graph of I vs V would be nonlinear
◦ Diodes are non-ohmic

Ohm’s Law not considered a fundamental law
of nature
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Length, cross-sectional area, material,
temperature
Length: ↑Length, ↑Resistance
X-section area: ↑Area, ↓Resistance
Material: Electrical conductivity determines
resistance
Temperature: ↑Temperature, ↑Resistance

Circuit components that regulate current flow
◦ May be a device (load) or component specificallydesigned to be resistor
◦ Usually cheaper to manipulate resistance than
potential difference for electronic devices

Materials that have zero resistance below a
certain temperature
◦ Critical temperature
◦ Normal resistance pattern for most temperatures


When temperature is at or below critical
temperature, resistance suddenly drops to
zero
Thousands of substances are
superconductors
◦ Aluminum, tin, zinc, lead

Conductivity not an indicator of
superconductivity
◦ Gold, silver, copper

When current established in superconductor,
current continues even when potential
difference is removed
◦ May persist without decay for years
◦ Electric currents produce magnetic effects

Meissner effect: interaction between a
superconductor and a magnet causes magnet
to levitate above superconductor

Current research seeks superconductors at
room temperatures
◦ Superconductivity found in some substances up to
150K
◦ Energy requirements for cooling materials to very
low temperatures is high
 Benefits from superconductivity often outweighed by
costs

As charges move through a system, they lose
energy due to collisions with other particles
and charge carriers
◦ Reach power source with zero potential energy
◦ Source must do work on charge to increase its
potential
 Potential increases by QΔV
 Power source loses equivalent amount of energy

Electric power is the rate of conversion of
electric energy
◦ Rate at which charge carriers do work

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Power is the rate at which charge carriers
convert electrical potential energy to
nonelectrical forms of energy
Formula: P = IΔV
◦ Describes rate at which charge carriers lose
electrical potential energy
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SI Unit: Watt (W)
Power dissipated by a resistor or load:
◦ P = I2R; P = (ΔV)2/R
◦ Joule heating: conversion of electrical energy to
internal energy in resistant material
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Power lines subject to joule heating or I2R
loss
Power companies want to minimize loss and
maximize energy delivered to consumer
◦ Can either decrease current or resistance
◦ Equation I2R states reducing current has more
impact on joule heating than reducing resistance

From P = IΔV, power can be delivered
through high current/low voltage or high
voltage/low current

Power companies deliver electrical energy at
very high voltages
◦ Transformers reduce voltages until power reaches
homes at about 120V